Re: [PATCH] ipc,sem block sem_lock on sma->lock during sma initialization

[Rik van Riel <riel@redhat.com>]

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On 11/23/2014 01:23 PM, Manfred Spraul wrote:
> Hi Rik,
> 
> On 11/21/2014 08:52 PM, Rik van Riel wrote:
>> When manipulating just one semaphore with semop, sem_lock only
>> takes that single semaphore's lock. This creates a problem during
>> initialization of the semaphore array, when the data structures
>> used by sem_lock have not been set up yet. The sma->lock is
>> already held by newary, and we just have to make sure everything
>> else waits on that lock during initialization.
>> 
>> Luckily it is easy to make sem_lock wait on the sma->lock, by
>> pretending there is a complex operation in progress while the sma
>> is being initialized.
> That's not sufficient, as sma->sem_nsems is accessed before
> calling sem_lock(), both within find_alloc_undo() and within
> semtimedop().
> 
> The root problem is that sma->sem_nsems and sma->sem_base are
> accessed without any locks, this conflicts with the approach that
> sma starts to exist as not yet initialized but locked and is
> unlocked after the initialization is completed.
> 
> Attached is an idea. It did pass a few short tests. What do you
> think?

This was my other idea for fixing the issue; unfortunately
I didn't think of it until after I sent the first patch :)

You are right that without that change, we can return the
wrong error codes to userspace.

I will give the patch a try, though I have so far been unable
to reproduce the bug that the customer reported, so I am unlikely
to give much in the way of useful testing results...

Andrew, feel free to give Manfred's patch my

Acked-by: Rik van Riel <riel@redhat.com>

It closes off more things than my patch did, in a similar way
(doing something before ipc_addid)

- -- 
All rights reversed
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